The invention relates generally to the field of component part inspection. More specifically, the invention relates to methods for inspecting gas turbine component parts after hydrogen fluoride cleaning.
Today's high strength braze repairs require very aggressive cleaning techniques in order to provide the level of cleanliness required to achieve optimum strength. These cleaning techniques do not discriminate between oxidation and contamination, and the elements that are designed to occupy the space between grain boundaries in a given alloy. Removal of these native elements from the grain boundaries is known as intergranular attack (IGA). Intergranular attack, also known as intergranular corrosion (IGC), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion. This attack is common in some stainless steels and nickel alloys.
Fluoride cleaning systems are used to remove unwanted oxides from surfaces and service induced cracks of turbine engine components, such as turbine blade airfoils, formed from nickel base superalloys prior to repairing the components. Hydrogen fluoride gas used in the cleaning treatment both depletes and intergranularly attacks the component surfaces and the exposed cracks, removing essential elements that form grain boundary carbides (i.e. grain boundary strengthening phases), leaving for some specific applications a desirable gamma layer on the surface and along the cracks which allow for capillary action during brazing. This depletion layer on the base superalloy is typically between about 0.0004 and 0.0012 inch deep. Presently acceptable levels of intergranular attack can be as high as about 0.011 inch deep in some alloys and some types of turbine airfoils.
Hydrogen fluoride cleaning used to clean parts results in IGA. Those components that can tolerate depletion and intergranular attack from the fluoride cleaning can be repaired and returned to service. If IGA is too deep, it can impact the integrity of the part by providing crack initiation sites.
Current methods for monitoring the extent of IGA involve processing coupons with parts to be cleaned. Coupons are cast pieces or pieces cut from a scrap component made of the same material as the cleaned parts. The coupons are sectioned to evaluate the level of IGA. Sample pieces may also be removed from the cleaned parts. The area(s) where the removed sample pieces were removed may have to be restored by puddle welding if the area cannot tolerate the missing material. The least attractive method is sacrificing a part to evaluate the level of IGA.
The coupon method has the drawback that the coupon has not experienced engine operation and may not have experienced the affects of any previous coating applications performed when assembled, or any coating removal prior to repair. Removing sample pieces may require welding to restore the part back to its original geometry. Further, welding of these superalloys is often difficult, resulting in heat affected zone cracking which results in reduced strength. Sacrificing one part examines only that part, not a number of parts.
Monitoring IGA is an important quality issue. An improved method for broad applications for repair procedures is desired.